Geomechanical Modelling the Evolution of a Connected Natural Fracture Network to Explain Fluid Flow Variations Across a Fractured Chalk-Marl Reservoir

Natural fractures are frequent in the chalk-marl successions of the Lower Cretaceous deposits constituting the Valdemar Field in the Danish Central Graben. However detailed knowledge on their evolution, nucleation and propagation has not previously been modelled and this study is the first to present numerical discrete fracture network models of the fracture patterns in the Lower Cretaceous strata of the Danish North Sea Basin. These strata are of heterogeneous nature and composed of interbeds of sedimentary facies comprising chalk, slightly marly chalk, marly chalk, chalky marlstone and marlstone. This lithological spectrum results in a range of mechanical properties. The Valdemar Field produces from three main reservoir intervals: the lower Tuxen, the middle-upper Tuxen, and the upper Sola. These intervals comprise a variety of sedimentary facies, and contain differing densities of natural fractures. The sedimentological subdivision of the reservoir correlates with mechanical variations within the different layers, and core studies have shown that the characteristics of the natural fractures vary according to the sedimentary facies. The three reservoir units therefore form the basis for the Discrete Fracture Network simulations, but additional simulations are also carried out on a single 10ft thick clean chalk bed within the upper Tuxen, which may act as a separate mechanical layer. The simulations are carried out by DFM Generator, a code for dynamic fracture modelling developed at Danish Offshore Technology Centre (DOTC) that simulates the growth of fracture networks based on the geomechanical properties of the lithology and the stress and strain history. This study presents geomechanical models of numerical simulations of discrete fracture networks modelled across selected reservoir zones in order to compare the nucleation, evolution, and propagation of fractures. The purpose of this study is to evaluate fracture patterns in the reservoir units and further to conduct a comparative study near two selected wells, one from an area of high productivity at the Jens High and one from an area of lower productivity at the Bo structure. The DFN models illustrate that the well with good production has well connected and dense fracture networks around it to facilitate fluid flow whereas the second well is adjacent to fractures that are widely spaced, less connected and primarily one directional. Thus, we propose that geomechanically based DFN models can act as a proxy of the subsurface conditions and indication of expected fracture growth areas in the reservoir